Armor plate of hardened steel has been used for many years to provide protection of objects against damage. Vehicles such as tanks, military sites, vaults, and safes, etc. have used steel armor plate to provide such protection.
In order to increase the protection provided, it has previously been proposed to use spaced layers of steel. For example, U.S. Pat. No. 1,548,441 Branovich discloses an armor protected fuel tank wherein a layer of wood and a layer of semi-cured rubber are positioned between a steel tank and an outer armor plate. U.S. Pat. No. 2,348,130 of Hardy, Jr. discloses spaced metal plates between which a layer of rubber is positioned with pockets in the rubber filled with abrasive material such as sand. U.S. Pat. No. 2,733,177 Meyer discloses an elastic cascading impact absorber wherein layers of armor are spaced with respect to each other by elastic material which is disclosed in preferred embodiment as being formed sheet metal springs. U.S. Pat. No. 4,455,801 Merritt discloses a lightweight vault wall wherein layers of metal, stainless steel and aluminum, cover spaced layers of plywood adjacent each of which is provided a layer of expanded metal mesh that is spaced from the other layer of expanded metal mesh by a foamed plastic core.
Two different basic types of armor plate are conventionally utilized at the present time. One type is high-hard armor that is extremely hard and thus capable of preventing penetration of penetrating type of projectiles. The other type is rolled homogenous armor that is somewhat softer than high-hard armor but is more ductile so as to prevent brittle fracture. Prior art references which disclose compositions and processing used in hardening of steel plates include: U.S. Pat. Nos. 774,959 Tresidder; 1,043,416 Giolitti; 1,079,323 Benthall; 1,097,573 Wales; 1,563,420 Johnson et al; and 1,995,484 Sullivan as well as the previously mentioned U.S. Pat. No. 2,733,177 Meyer.
In order to decrease weight, armor plate and the like have previously included holes such as illustrated by U.S. Pat. No. 3,763,838 of Butterweck et al which discloses a protective shielding for vehicles. While circular holes such as disclosed by Butterweck et al or slots are the easiest to produce in armor by punching, such shapes have ballistic gaps that reduce the protection provided. Similarly, square holes which will provide the lowest weight also have ballistic gaps that reduce the protection provided.
Other prior art references disclosing armor plate or the like include U.S. Pat. Nos.: 45,536 Terwilliger et al; 874,729 DeBolula; and 4,178,859 Seiz et al.